The final result of screen printing is determined by the technology used, so every tool, skill, formula and operator has a more or less influence on it, even the most obscure ink scraper in screen printing Played an important role. Whether it is manual printing or mechanical printing, the doctor blade is one of the most used tools in ink transfer.
In addition to using the doctor blade, there are other methods to force the ink through the screen, such as the vacuum pressure method or the roller method, but the doctor blade is the most commonly used tool. The squeegee consists of several parts: a wooden board with a groove as a handle; a rubber blade inserted into the groove of the wooden board to protrude the groove; and a fixing device. The most commonly used squeegee is made of two wooden boards. The method of connecting the doctor blade handle to the doctor blade is also different.
Squeegee handles are mostly wooden, but also made of aluminum. The groove handle can be made of hardwood or softwood, but it must be dried and without defects. Cork (such as white pine) is used because it not only has sufficient strength, but also lighter, can make large-sized handles. This is because the length of the doctor blade can reach about 72 inches (183 cm), so its weight is very important. Lighter doctor blades are easier to use.
The handle of the squeegee should be in contact with the operator's hand, so the corners of the handle should be rounded to prevent hand rubbing. The rubber blade must fit the groove of the handle well. If it does not fit well, ink of different colors will dry in the gap between the blade and the groove of the handle, affecting the color of the printing ink. Some operators apply shellac to the position where the blade touches the groove of the handle to prevent the ink from drying and make it easy to clean. In large screen printing workshops that use three and four colors for printing, different squeegees are used for each color, and they are cleaned after use to keep the same color. The wooden part of the squeegee should be sanded with sandpaper to ensure that there are no rough spots.
Rubber can also be used to make blades, which can be polyurethane or other types, but it must have good moisture resistance, corrosion resistance and heat resistance. It must be resistant to animals, minerals, vegetable oils and fats. It should also be able to withstand the oxidation effects of the metal components contained in the ink. The rubber should not be affected by benzene, alcohol, lacquer, enamel, screen cleaning solution and film solution. Artificial rubber has better fire resistance and better printability than natural rubber. Natural rubber has a short life, is easily deformed, and is easily dissolved by the solution. The source of rubber raw materials is the same as all pigments and composites. Rubber blades have various hardnesses. For screen printing, 45 to 80 hardness blades are generally used. Generally speaking, suppliers have three blades: soft, medium and hard. 45 ~ 50 hardness is extra soft blade, 75 ~ 80 is extra hard blade.
The blade edge determines the uniformity of the ink and the thickness of the deposit. The blade edge must be parallel to the printing surface. The blade should fit well in the groove or between two pieces of wood, and fixed with wood wire nails. To fix the wood wire nails one to four inches away, cross the nails, first through one side of the board and then into the other side. The thickness of the blade is about 1/4 to 1/2 inches (0.63 to 1.27 cm); the height is about 1.5 to 3 inches (3.8 to 7.6 cm). The thickness, height and length of the blade depend on the handle size of the doctor blade and the length of the doctor blade. Although some printers can use tools to make squeegees by hand, they are not practical after all. The best squeegee blades suitable for various purposes are available from screen printing suppliers and are cheap. Its price varies with the length of the squeegee line. Intact doctor blades are available in various lengths, and the blade and handle can be purchased separately.
The length of the doctor blade is determined by the work to be completed and the size of the screen frame used. It is recommended that the squeegee used is 2 to 3 inches (5 to 7.6 cm) smaller than the screen size, but it must cover the entire pattern to be printed in one stroke.
In order to maintain the gap between the two printings so that the squeegee and the ink are not in contact, and give the staff a breathing time, a support bar needs to be inserted between the squeegees. There is also a method of inserting a support rod in the vertical direction of the doctor blade surface. The doctor blade is usually controlled with two hands. When the doctor blade only needs one hand to control, the other hand can be free to rest.
The squeegee, like all tools, has some points to be aware of. This is mainly to maintain the sharpness and straightness of the blade. Its sharpness and straightness can be tested with a ruler or guideline. When the blade is dull, damaged and uneven, the doctor blade needs to be handled by the staff or the wire mesh supplier to restore it to sharpness and straightness. If the squeegee is not sharp or straight, the ink layer of the printed product is not flat, and workers with poor skills may sometimes think that it is the cause of the ink or screen cloth. When using a screen printer for printing, the flatness of the blade of the squeegee must be regularly checked to ensure the uniformity of the ink layer of the printed product.
Use the sharpening jig or special machine to sharpen the blade. The production of the jig is very simple and can be made of wood. The length is 1 ~ 3 feet (0.305 ~ 0.914 meters). The abrasive medium of the sharpening jig can use No. 0 sandpaper, or emery cloth with 80 ~ 120 coarse gravel, or new (or old) sandpaper. The method of sharpening is to hang the squeegee in a vertical position and pull the abrasive material back and forth until the blade becomes smooth. If there is a belt sander or disc sander, it will save a lot of work for workers. The speed of the sander is 1000 ~ 1300 rpm. If the speed exceeds 1300 rpm, the rubber may be melted because most synthetic rubbers are thermoplastic materials and become soft when heated. If this happens, you can add a small amount of water to cool it to prevent decomposition and softening. In addition, the rubber should be lightly pressed so that it can be rubbed without melting.
Another method of sharpening the blade is to cut a part of the blade of the blunt blade with a knife sharper than the blade, a manual cutter or a mechanical cutter. If the blade or paper knife is wiped with a damp cloth, it will be easier to cut off the rubber blade. If the blade is damaged so badly that it cannot be flattened, you have to replace it with a new blade.
The squeegee must be kept clean and cleaned with a cleaning agent that cleans the screen. Dirty squeegee blades are more difficult to clean after sticking ink. Moreover, the dried ink may react with various colors of ink after printing. The squeegee should be stored for two hours before cleaning, so as to completely evaporate the solution. In addition, if the storage time is longer, it is recommended to use French lime powder on the blade to protect the blade. Generally, it should be hung up or placed on the knife holder during storage.
Sichuan Soper Science & Technilogy Co., Ltd , https://www.Soper.com.cn